1. Field of the Invention
The present invention relates to a method for preventing a sewing thread from raveling from a sewn end of a seam, in the sewing of a cloth by a double chain stitch sewing machine, which forms a double chain stitch by needle threads passed through needles moving vertically, and a looper thread passed through a looper moving forward and backward between a forward position and a backward position across the needles, and to an apparatus used for realizing such method.
2. Description of the Prior Art
When sewing a cloth with this kind of double chain stitch sewing machine, if a double chain stitch as indicated by the seam symbol 406 of Federal Standard No. 751a (JIS, Japanese Industrial Standard, L 0120), for example, is formed in the cloth, the sewing thread of this seam is raveled sequentially when the looper thread is pulled from the sewing end, and the quality of the sewn product is lowered.
To prevent the sewing thread of the double chain stitch from raveling from the sewn end, hitherto, various methods have been known, including (a) a general method of sewing by a condensed stitch at several stitches just before the sewn end, (b) a method of making the sewing thread hard to ravel by changing the tension balance of the sewing thread at the sewn end as disclosed in Japanese Laid-open Patent No. 1-317475 (Japanese patent Publication No. 6-102107) and Japanese Laid-open Patent No. 5-208082 (corresponding to U.S. Pat. No. 5,381,745), and (c) a method, as disclosed in Japanese Laid-open Patent No. 6-233877, of slidably disposing a looper thread hook for holding slidably the looper thread behind the looper of a double chain stitch sewing machine. This method comprises the steps of stopping the operation of the sewing machine once immediately before the sewing finish holding the looper thread by the looper thread hook, resuming the operation of the sewing machine in this state to sew one stitch, and cutting both needle threads and looper thread after the sewing. Then, as the sewing operator removes the cloth from the sewing machine while holding the looper thread by the looper thread hook, the entangling state of the needle thread and looper thread is varied so that the sewing thread may be hard to ravel.
Of the conventional methods for preventing thread from raveling, however, in the above methods (a) and (b), raveling of the sewing thread from the sewn end is not prevented securely. In the method (c), a long end of the cut thread hangs loose from the cloth, and the appearance of the sewn product is poor, or to improve the appearance, the manual labor of clipping the loose end of the thread off the cloth after finishing the sewing process is required.